1. Technical Field
The present disclosure relates to a display device, and more particularly, to pixel in a a display device and a control method thereof.
2. Discussion of the Related Art
In a display device including an organic light-emitting diode (OLED), which is a self-emitting element, respective pixels can perform a grayscale presentation by controlling a driving current running through the OLED. The brightness deviation may occur in a display device due to the non-uniformity, which can be caused by process errors, and so forth, of electrical characteristics, such as the threshold voltage and mobility of the TFT, especially the driving TFT, in the respective pixels.
As a solution to the above-mentioned problem, the non-uniformity characteristic of the brightness due to the change of the electrical characteristics (e.g., the threshold voltage and mobility) of the driving TFT may be cured by sensing the change of the electrical characteristics of the driving TFT in the respective pixels, and by properly compensating for input data according to the sensing result. This solution is referred to as an “external compensation” scheme. A pixel, to which the external compensation scheme may be applied, may include a data TFT for receiving data, a light-emission control TFT for controlling the current amount of the OLED, and a sensing TFT for sensing, as well as the driving TFT.
FIG. 1 is a circuit diagram illustrating a basic structure of a pixel in which an external compensation scheme is adopted according to a related art. FIG. 2 is a timing diagram illustrating an operation of the pixel shown in FIG. 1.
With reference to FIGS. 1 and 2, the related art pixel includes a light-emission control thin film transistor (TFT) M1, a driving TFT M2, a data TFT M3, a sensing TFT M4, a capacitor Cs and an organic light-emitting diode OLED.
The light-emission control TFT M1 receives a light-emission control signal EM at its gate, receives a power voltage VDD at its drain, and is coupled to the driving TFT M2 at its source. The light-emission control TFT M1 stays turned on and allows current to flow through the driving TFT M2 when the light-emission control signal EM is enabled.
The driving TFT M2 is coupled to a first node “a” at its gate, is coupled to a second node “b” at its source, and is coupled to the light-emission control TFT M1 at its drain. When turned on, the driving TFT M2 controls a driving current to flow through the OLED. As the amount of the driving current becomes greater, the light emission amount of the OLED becomes greater, which makes the grayscale presentation possible. The driving current is related to the gate-to-source voltage VGS between the gate and source of the driving TFT M2. As the voltage VGS between the gate and source of the driving TFT M2 becomes greater, the amount of the driving current becomes greater. The data TFT M3 receives a scan signal “scan” at its gate, receives a data signal Data at its source, and is coupled to the first node “a” at its drain. The data TFT M3 transfers the data signal Data to the first node “a” when the scan signal “scan” is enabled.
The sensing TFT M4 receives a sensing signal “sense” at its gate, receives a reference voltage Ref at its source, and is coupled to a third node “c” at its drain. The third node “c” is electrically the same as the second node “b.” The sensing TFT M4 senses the voltage change of the third node “c” when the sensing signal “sense” is enabled. For example, the sensing TFT M4 senses the threshold voltage of the driving TFT M2 by sensing the voltage of the third node “c”.
The capacitor Cs is coupled between the first node “a” and the second node “b”. The capacitor Cs maintains the voltage difference between the first node “a” and the second node “b” of the driving TFT M2 (i.e., the voltage difference between the gate and the source of the driving TFT M2). The OLED is coupled to the third node “c” at its anode, is coupled to a ground voltage VSS at its cathode, and includes an organic compound between the anode and the cathode.
In the above example, each of the light-emission control TFT M1, the driving TFT M2, the data TFT M3, and the sensing TFT M4 is an N-type metal oxide semiconductor (NMOS) TFT. However, any of the TFTs may be a P-type metal oxide semiconductor (PMOS) TFT, in which case, the respective source/drain terminals would be reversed from the above description.
During a first time period T1, the scan signal “scan” and the sensing signal “sense” are enabled while the light-emission control signal EM is disabled. During the first time period T1, the data TFT M3 turned on by the enabled scan signal “scan” transfers the data signal Data from a fourth node “d” to the first node “a”. The capacitor Cs maintains the gate-to-source voltage VGS between the gate and source of the driving TFT M2.
The sensing TFT M4 is turned on by the sensing signal “sense” being enabled, and transfers the reference voltage Ref from a fifth node “e” to the third node “c”. The light-emission control TFT M1 stays turned off due to the light-emission control signal EM being disabled, and blocks the driving current from flowing from the driving TFT M2 to the OLED. During the first time period T1, the data signal Data is provided for the grayscale presentation.
During a second time period T2, the scan signal “scan” and the sensing signal “sense” are disabled while the light-emission control signal EM is enabled. The light-emission control TFT M1 is turned on by the enabled the light-emission control signal EM, and the driving TFT M2 is also turned on by the voltage maintained in the capacitor Cs. Thus, the driving current flows through the OLED in proportion to the voltage maintained in the capacitor Cs. The second time period T2 is a light-emission period of the OLED, or a “display-on” period.
During a third time period T3, the scan signal “scan” and the light-emission control signal EM are disabled, while the sensing signal “sense” is enabled. Therefore, the data TFT M3 and the light-emission control TFT M1 are turned off, while the sensing TFT M4 is turned on. The sensing TFT M4 senses the voltage change of the third node “c” in response to the enabled sensing signal “sense” during the third time period T3 when the turned-off light-emission control TFT M1 blocks the driving current from flowing from the driving TFT M2 to the OLED. Although not illustrated, the sensed voltage is compared and a compensated voltage is obtained by a separate circuit, and thus the compensation operation may be completed.
According to the related art described above, the light-emission control signal EM and the light-emission control TFT M1, which control the time period for the light-emission of the OLED, are required to block the driving current from flowing through the OLED during the time period when the light emission is not required. Also, the sensing signal “sense” and the sensing TFT M4 controlled by the sensing signal “sense” are required for the external compensation scheme. A plurality of TFTs for respective functions in an area of a pixel limits a number of pixels in the size-limited display device.
It is a recent trend that the pixel size required for a high density display has been shrinking. A TFT for the compensation is required to cure the brightness deviation and to improve image quality. The highly dense and smaller pixel is also required to follow the recent trend. Accordingly, what is needed is a technology for compensating for a pixel without increasing the pixel size.